The Fae created a barrier array that should create the wall of vines all along the boundary that was marked by the poles, which would attack the undead that made it that far and throw them back.
It wasn't a solid wall, but more of a row of vines that would extend upwards when needed, so the defenders could still shoot over it freely. The Unholy Fire barrier was directional, so it could be fired through, though it would light the bullets on fire as they passed.
It should be a great combination, and Wolfe was actually looking forward to seeing what the tentacles would look like covered in dark blue, Unholy Flames. He could bet that nobody else would be expecting that, though perhaps a few of the Fae had tried merging tentacles and fire in the same defensive array before.